Application Load Balancer (ALB) in AWS - Step-by-Step Execution

Process Flow - Application Load Balancer (ALB)

Client sends HTTP request

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ALB receives request

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ALB evaluates listener rules

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Select target group based on rules

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Forward request to healthy target instance

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Target instance processes request and responds

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ALB sends response back to client

The ALB receives client requests, checks rules to decide which server group to send to, forwards the request to a healthy server, and returns the response to the client.

Execution Sample

AWS

resource "aws_lb" "example" {
  name               = "example-alb"
  internal           = false
  load_balancer_type = "application"
  subnets            = ["subnet-123", "subnet-456"]
}

This code creates an Application Load Balancer named 'example-alb' in two subnets, ready to route HTTP requests.

Process Table

Step	Action	Input/Condition	Result/Output
1	Client sends HTTP request	Request to ALB DNS	Request arrives at ALB
2	ALB listener receives request	Listener on port 80 active	Listener accepts request
3	ALB evaluates listener rules	Rule: path /api/* -> target group A	Request matches /api/* rule
4	ALB selects target group	Target group A has 3 healthy instances	Target group A chosen
5	ALB forwards request	Select healthy instance from target group A	Request sent to instance i-12345
6	Instance processes request	Instance i-12345 receives request	Instance returns HTTP 200 response
7	ALB sends response to client	Response from instance	Client receives HTTP 200 response
8	End	No more requests	Waiting for next client request

💡 Execution stops after response is sent back to client and ALB waits for new requests.

Status Tracker

Variable	Start	After Step 3	After Step 4	After Step 5	Final
Request Path	/	/api/users	/api/users	/api/users	/api/users
Selected Target Group	None	None	Target Group A	Target Group A	Target Group A
Target Instance	None	None	None	i-12345	i-12345
Response Status	None	None	None	None	200 OK

Key Moments - 3 Insights

How does the ALB decide which target group to send the request to?

The ALB checks listener rules against the request path or host. In the execution table at Step 3, the request path '/api/users' matches the rule '/api/*', so target group A is selected at Step 4.

What happens if no target instances are healthy?

If no healthy targets exist in the selected target group, the ALB cannot forward the request, resulting in an error response. This is not shown in the current execution but would stop forwarding at Step 5.

Why does the ALB wait after sending the response?

After sending the response (Step 7), the ALB waits for new client requests to handle. This is shown in Step 8 where it is idle, ready for the next request.

Visual Quiz - 3 Questions

Look at the execution table, at which step does the ALB select the target group?

A Step 2

B Step 4

C Step 6

D Step 7

💡 Hint

Check the 'Action' column for 'ALB selects target group' in the execution table.

According to the variable tracker, what is the 'Response Status' after Step 7?

A None

B 404 Not Found

C 200 OK

D 500 Internal Server Error

💡 Hint

Look at the 'Response Status' row under the 'Final' column in the variable tracker.

If the request path was '/home' instead of '/api/users', how would the selected target group change?

A Target Group A would still be selected

B No target group would be selected

C A different target group would be selected if rules exist

D The ALB would forward to all target groups

💡 Hint

Refer to Step 3 in the execution table where rules are matched based on the request path.

Concept Snapshot

Application Load Balancer (ALB):
- Receives client HTTP/HTTPS requests
- Uses listeners and rules to route requests
- Forwards requests to healthy targets in target groups
- Supports path and host-based routing
- Returns responses from targets back to clients
- Automatically handles scaling and health checks

Full Transcript

An Application Load Balancer (ALB) receives HTTP requests from clients. It listens on specified ports and evaluates rules to decide which target group should handle the request. The ALB forwards the request to a healthy instance in the chosen target group. The instance processes the request and sends back a response. The ALB then returns this response to the client. After completing a request, the ALB waits for new incoming requests. This process ensures efficient and reliable routing of web traffic to backend servers.